I learned a lot of stuff at a restoration shop I worked at back in the seventies, that was one of the things that I have never forgot. We also had a tool to paint them on. It was a stand with a old spindle on it with a hub. The hub had metal rods welded off the top of the studs on the hub. Those rods were about 7/16" round and had a knotch ground in it at the top on the outer side. You would flex the rods togther and slip the rim over them at the lug holes. The rods pressure going out on the edge of the rim right at the knotch would hold the rims on. You could spin the rim as you painted it. Once the tire is mounted you just roll the wheel on the paper masked tire to get all the edges.
That guy was full of some great ideas, we had a rotating frame painting rack years before I ever saw one anywhere else.